The present invention relates to a communication method based on CDMA (Code Division Multiple Access) and a communication system, more particularly, to a CDMA communication method and a spread spectrum communication system both suitable for packet transmission.
In CDMA communication, a signal to be transmitted is multiplied by a spreading code to spread the resultant signal over a wide band for communication and spreading codes having different patterns are assigned to different channels to multiplex many signals for communication. The CDMA communication is excellent in security and high in frequency usage efficiency and stability, finding suitable applications in mobile communication and cable transmission, some of them currently being put into practical use.
The inventors hereof proposed a mobile communication system for efficiently transmitting data packets based on such CDMA communication (refer to U.S. patent application Ser. No. 08/690819). In this proposition, the wireless channels between a base station and a plurality of mobile stations have a reservation channel for transmitting reservation packets from the mobile stations to the base station, a response channel for transmitting response packets from the base station to the mobile stations, and a data traffic channel for transmitting data packets along the reverse link running from the mobile stations to the base station and the forward link running from the base station to the mobile stations.
Before starting data transmission, each of the mobile stations with transmitted data ready requests the base station for the allocation of the data traffic channel and a time slot to be used. As shown in FIG. 12, this request (the request for reservation) is made by transmitting the reservation packet including a mobile station ID to the reservation channel. It should be noted that spectrum-spreading the reservation packet by a spreading code allows each mobile station to transmit the reservation packet any time.
According to the reservation packet received at the reservation channel, the base station assigns, for each mobile station, the data traffic channel and the time slot to be used (scheduling shown in FIG. 12) and transmits the information thereof to the response channel as a response packet. Receiving the response packet including own ID, each mobile station transmits the data packet in the time slot specified in the data traffic channel specified in this response packet.
In transmitting a data packet from the base station to a mobile station, the base station transmits a response packet for reception control specified with the ID of the destination mobile station, a data traffic channel, and a time slot to the above-mentioned response channel. Receiving the response packet for reception control including own ID, each mobile station receives a data packet in the specified time slot on the data traffic channel specified in this response packet.
The above-mentioned reservation channel, response channel, and data traffic channel each have a predetermined frequency bandwidth assigned to the mobile communication system to form the system frequency band. These plurality of channels are multiplexed by spreading codes having different patterns for these channels and a common chip rate, resulting in the same bandwidth throughout the channels.
On the other hand, the information to be transmitted by each mobile station are various; for example, the data includes low-bit-rate audio information and facsimile information and high-bit-rate data and images. Conventionally, these various pieces of information are transmitted by changing the number or length of packets according to transfer rates used. Therefore, in the prior-art technology, various pieces of data can be integrated into one unit but it is difficult to handle each piece of information independently, thereby lowering the flexibility in communication service.